1. Field of the Invention
The invention relates to a spark plug which is improved to optimally located a firng portion within a combustion chamber upon mounting the spark plug on a cylinder head of an internal combustion engine.
2. Description of Prior Art
In recent years, a lean burn type engine and high output engine have been introduced chiefly with an aim to obtaining a high efficiency. In these types of the engines, when an air-fuel mixture injected into a combustion chamber is ignited by a spark plug, it often burns in laminar form because a density distribution of the air-fuel mixture greatly varies within the combustion chamber. In order to cope with the situation, it is necessary to determine where a firing portion of the spark plug is to be optimally located within the combustion chamber. On the other hand, due to the air-fuel mixture thickly concentrated around the firing portion of the spark plug, the carbon fouling is likely to deposit on a front end of an insulator.
In order to improve the carbon fouling resistance, a published Japanese application No. 5-46673 discloses a spark plug in which carbon deposit is burningly removed by thinning a front end of an insulator nose in order to quicken its temperature rise with a minimum heat capacity.
In each of a laying-open Japanese application No. 60-235379, published Japanese application No. 3-41951 and published Japanese application No. 56-47915, a spark plug is disclosed to improve the carbon fouling resistance and heat resistant property by mainly determining an extension length protracted from an inner wall of the combustion chamber to a front end of a center electrode.
However, it is found that these types of the spark plugs have a enough room for further improvement from the points of the carbon fouling resistance and heat resistant property as a result of carrying out an experimetal test with the above prior art spark plugs mounted respectively on the high efficient engine.
Therefore, it is a main object of the invention to provide a spark plug which is capable of obtaining a good carbon fouling resistance without sacrificing a favorable ignitability when mounted on an internal combustion engine which tends to carbon smolder an insulator.